1. Field of the Invention
The present invention relates to thick film conductive pastes for microelectronic circuitry to form electrodes; electrical connections between resistors, capacitors or other electronic components and integrated circuits; or interconnections of multilayer ceramic modules; or the like.
More particularly, the present invention relates to conductive pastes in which any toxic lead frit system is not introduced for providing thick film conductors.
2. Description of the Prior Art
Conventional conductive pastes typically comprise; conductive particles such as noble metals, copper or nickel; inorganic binders such as glass and metal oxide additives; and an organic medium. Pastes are applied on insulating ceramic substrates such as alumina and on dielectrics and fired at high temperatures, e.g., 600.degree.-1000.degree. C.
For example, in the process of manufacture of thick film multilayer circuits, conductive pastes are fired usually at 800.degree. C. to 900.degree. C. and refired many times because thick film multilayer circuits are fabricated through repetition of printing and firing conductors, dielectrics, resistors and encapsulants.
Most existing thick film pastes contain lead compounds in the form of PbO-containing glass frit, PbO, Pb.sub.3 O.sub.4, etc., as an essential ingredient. Experimental studies have established that lead compounds are essential for the strong adhesion of thick film conductors to alumina and to dielectrics as well as for good solder leach resistance.
However, the use of lead compounds are recently restricted both from the environmental point of view and due to their high health hazards, and will be banned in some areas in the near future.
It has been proposed to use lead-free glass frits, for example, containing Bi.sub.2 O.sub.3 as a main ingredient. However, these glass frits have a low glass transition temperature (Tg) of lower than 550.degree. C. So, it is estimated that conductive pastes with these glass frits are sintered overly when fired at a firing temperature of 800.degree. C. to 900.degree. C., for about 10 minutes. This process frequently results in poor thick film properties.